The ignition period required in an engine generally differs according to the engine running conditions, and a basic ignition period is therefore usually set to an MBT (Minimum spark advance for best torque) which varies according to engine load and engine rotation speed. All additional correction of the basic ignition period is made for engine cooling water temperature so as to improve warm-up performance immediately after starting the engine or to prevent knocking on high load.
An ignition period controller which controls knocking by applying a delay correction related to engine cooling water temperature to the ignition period is for example disclosed in Tokkai Sho 59-201970 published by the Japanese Patent Office.
To prevent knocking, it would be appropriate to delay the ignition period under running conditions in which knocking tends to occur easily such as when the engine is at a high temperature, but this delay also leads to a decrease of engine torque. It is therefore necessary to limit the delay correction to a minimum.
For this purpose, this controller has been made to prevent knocking while maintaining engine output performance by applying a delay correction to the ignition period based on one of two characteristics, i.e. a delay characteristic where the delay amount increases depending on increase of engine temperature, and a delay characteristic where the delay amount increases depending on increase of engine load. Of these two characteristics, the delay characteristic is chosen which gives the lesser delay amount.
In exhaust purification systems employing a three-way catalyst which oxidizes HC and CO while simultaneously reducing NO, the desired exhaust purification efficiency is obtained only when the catalyst has reached an active state above a predetermined temperature. In other words, the exhaust purification efficiency is low, for example when the engine has a cold start, until the catalyst temperature rises.
For example, in the ignition period controller disclosed in Jikkai Sho 57-75173 published by the Japanese Patent Office, the ignition period is delayed so as to increase exhaust temperature when the engine cooling water temperature is low, and the catalyst is activated as soon as possible after start-up. This also helps the engine to warm up.
However, as a uniform delay correction amount is normally applied to the basic ignition period depending on the cooling water temperature, torque insufficiency or sturge may occur, especially when the engine is on high load, if the running load increases during the delay period. For example, if the engine accelerates before it has completely warmed up, or if it is run on high load, the ignition period is too much delayed even if the basic ignition period is adjusted according to the load, with result that the output torque decreases, fuel costs increase and performance deteriorates.
If on the other hand the lesser delay amount is chosen from two delay characteristics of the ignition period as in the case of the Tokkai Sho 59-201970, decrease of output torque can be considerably suppressed. However, even in this controller, the ignition period is delayed the greater the load or the higher the temperature of the cooling water, so prevention of knocking is still being emphasized at the expense of engine performance. In this controller too therefore, the deterioration of acceleration performance during warm-up is inevitable.
Moreover, after depressing the accelerator pedal to a certain opening in order to accelerate the vehicle, when for example the acceleration is continued with the same accelerator opening, intake negative pressure varies with the engine rotation speed even if the accelerator opening does not vary. If the intake negative pressure is chosen as representative of engine load in order to control the ignition period, the delay amount of the ignition period varies depending on the change of intake negative pressure. The output torque therefore fluctuates while the accelerator opening is constant, and in the case of an automobile engine, this fluctuation deteriorates the driving response of the vehicle.